Intercropping grain crops with green manure under reduced chemical nitrogen improves the soil carbon stocks by optimizing aggregates in an oasis irrigation area

Enhancing soil organic carbon (SOC) stocks is a key aspect of modern agriculture, but whether this can be achieved by incorporating legume green manure crops in cereal production to substitute synthetic N fertilizers is unknown. This study used a six-year (2017–2022) field study to explore the impacts of intercropping green manure with maize and reducing nitrogen fertilization on SOC stocks, while specifically focusing on the relationship between aggregate composition and carbon sequestration. Maize intercropped with common vetch (M/V), maize intercropped with rapeseed (M/R), and sole maize (M), were each tested at conventional (N2, 360 kg ha–1) and reduced (N1, 270 kg ha–1, 25% reduced) N application rates. Soil was sampled in 2020, 2021, and 2022. Compared with sole maize, intercropping with green manure (M/V and M/R) significantly increased SOC stocks which compensated for any negative effect due to the 25% reduction in N application. Based on 3-year averages, intercropping with M/V and M/R increased the SOC content compared to sole maize (M) by 12.1 and 9.1%, respectively, with intercropping further mitigating the negative impact of reduced nitrogen application. There was no significant difference between M/V and M/R. The SOC content at N1 was reduced by 9.3–10.5% compared to that at N2 in sole maize, but the differences in SOC stocks between N1 and N2 were not significant in the intercropping patterns (M/V and M/R). The intercropped M/V and M/R showed 20.9 and 16.3% higher SOC contents compared to sole maize at N1, with no differences at N2. Intercropping green manure led to a 5.3% greater SOC in the 0–20 cm depth soil in 2022 compared to that in 2020, due to the cumulative effect of two years of green manure intercropping. Intercropping green manure (M/V and M/R) increased the proportion of macroaggregates (>0.25 mm) and aggregate stability while reducing the proportion of microaggregates compared to sole maize under the N1 application. Structural equation modeling indicated that cropping patterns and nitrogen application levels mainly affect SOC indirectly by regulating the composition of macroaggregates and aggregate organic carbon (AOC). Correlation analysis further revealed that the composition of macroaggregates is significantly and positively correlated with the SOC content (R²=0.64). In addition, intercropping green manure can maintain high crop yields by increasing SOC under reduced chemical nitrogen application. The results of this study show that intercropping green manure with grain crops can be a viable measure for increasing SOC sinks and maize productivity by optimizing the aggregate composition with reduced N application in the Hexi Oasis Irrigation Area.